Safely ingestible batteries and methods

ABSTRACT

A battery for use in electronic devices and which is safely ingested into a body and a related method of making the battery. The battery includes an anode, a cathode and a quantum tunneling composite coating. The quantum tunneling composite coating covers at least a portion of at least one of the anode or the cathode and provides pressure sensitive conductive properties to the battery including a compressive stress threshold for conduction. The compressive stress threshold may be greater than a pre-determined applied stress in a digestive tract of the body in order to prevent harm if the battery is ingested. The battery may include a waterproof seal that extends between the quantum tunneling composite coating and a gasket separating the anode and cathode to inhibit the battery from short circuiting in a conductive fluid below the compressive stress threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/774,984, filed Sep. 11, 2015, which is a U.S. national stageapplication of International Application Number PCT/US2014/020537, filedMar. 5, 2014, which claims the benefit of the filing date of U.S.Provisional Application No. 61/778,928, filed Mar. 13, 2013, the entirecontents of which are incorporated by reference herein for all purposes.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support awarded bythe following agencies: NIH Grant Nos. DE013023 and GM086433. The UnitedStates government has certain rights in this invention.

BACKGROUND

This disclosure relates to batteries. In particular, this disclosurerelates to batteries with a coating that serves as a safety coating andprovides pressure-sensitive conduction.

Billions of button batteries (also known as button cells) are sold eachyear to power portable electronic devices including, for example, smallPDAs, musical greeting cards, glucometers, watches, virtual pet devices,hearing aids, and laser pointers. Tragically, accidental ingestion ofthese small batteries caused more than 40,400 children under the age of13 to visit hospital emergency rooms, with 14 battery-related deaths inchildren 7 months to 3 years of age between 1997 and 2010 in the UnitedStates alone. As manufacturers create more powerful button batteries insmaller casings, button battery ingestion and injury is on the rise, andthe increase in battery power yields a corresponding increase inseverity of injuries and mortality resulting from button batteryingestion. Though safety standards now regulate locked batterycompartments in toys, minimal technological development has taken placeat the level of the battery to limit injury, particularly batteries inthe greater than or equal to 20 millimeter format which are recognizedas leading causes of complications if ingested. In addition to children,especially those under the age of five, an increasing number of seniorsingest button batteries after mistaking the button batteries for pills,particularly as button batteries are ubiquitous in devices usedfrequently by seniors, such as hearing aids. Furthermore, countless petsingest button batteries each year.

Gastrointestinal (GI) obstruction is typically the first clinicalsymptom of button battery ingestion. However, button battery ingestionis more severe than ingestion of comparably sized objects, such ascoins, due to damage by short circuit currents. Current flow inconductive GI fluids can cause electrolysis, generate hydroxide ions,and create long-term tissue damage in the digestive tract. Shortcircuiting of ingested button batteries has caused acute injuriesincluding esophageal and GI perforations, trachea-esophageal fistulae,arterio-esophageal fistula leading to death, esophagealstenosis/stricture, chemical burns, as well as vocal cord paralysis.Case studies have shown that GI perforation in humans can occur as soonas five hours after battery ingestion. In pets, severe GI damage occurseven more quickly, with reports of transmural esophageal necrosis withinone hour of ingestion in dogs and within two to four hours in cats.

Additionally, the short circuit current can damage the polymer gasketthat separates the anode and cathode. Once the gasket is impaired orcompromised, the contents of the button battery, including toxic metalssuch as cadmium, lead, mercury and lithium, may be released into thebody. The release of button battery contents poses both the acute andlong-term health risks associated with heavy metal ingestion.

SUMMARY OF THE DISCLOSURE

Given the various problems outlined above if batteries are ingested, aneed exists for an improved battery. In particular, there is a need fora battery that, if ingested, can limit the external electrolyticcurrents responsible for damage to intestinal tissue as well as inhibithazardous materials from entering the body.

The disclosed battery overcomes the aforementioned drawbacks byproviding an ingestible battery with pressure-sensitive orpressure-gated conductive coating to prevent damage to intestinal tissueand to prevent the battery from releasing harmful contents into the bodyif ingested. In particular, a waterproof, pressure sensitive, quantumtunneling composite coating (QTCC) may serve as an electricallyinsulating barrier in the intestinal environment under the appliedstress of a digestive tract while still being conductive in standardbattery housings in which the stress applied to the battery in thehousing exceeds the stress applied in the digestive tract.

According to one aspect, a battery is disclosed for use in electronicdevices. The battery can be safely ingested into a body (e.g., a humanbody) that exerts a pre-determined applied stress in a digestive tractof the body. The battery includes an anode, a cathode, and apressure-sensitive coating. The pressure-sensitive coating covers atleast one of the anode and the cathode and provides pressure sensitiveconductive properties including a compressive stress threshold. When astress above the compressive stress threshold is applied to thepressure-sensitive coating, the pressure-sensitive coating is placed inan electrically conductive state. In order to avoid harm if the batteryis ingested, the compressive stress threshold for conduction is greaterthan the pre-determined applied stress associated with (i.e., appliedby) the digestive tract of the body.

The pressure-sensitive coating may be a quantum tunneling compositecoating. When a stress above the compressive stress threshold is appliedto the quantum tunneling composite coating, the quantum tunnelingcomposite coating may be placed in a conductive state in which electronsare able to tunnel through the quantum tunneling composite coating. Whenno stress or a stress below the compressive stress threshold is appliedto the quantum tunneling composite coating, the quantum tunnelingcomposite coating may be said to be in an insulating state in whichelectrons are unable to quantum tunnel through the quantum tunnelingcomposite coating.

The quantum tunneling composite coating may include a polymer matrixwith conductive microparticles suspended therein. Collectively, thepolymer matrix and the conductive microparticles provide the pressuresensitive conductive properties for the quantum tunneling compositecoating. The polymer matrix is elastically deformable to alter thespacing of the suspended conductive microparticles relative to oneanother. In an unstressed state, the conductive microparticles aresufficiently spaced from one another, such that no conduction betweenthe microparticles occurs, either via tunneling or direct conduction.However, under an applied stress that places the polymer matrix in astressed state, the polymer matrix is at least temporarily elasticallydeformed in order to alter the spacing of the microparticles relative toone another. This decrease in spacing between the microparticles permitsthe tunneling of electrons through the polymer matrix in such a way asto make the quantum tunneling composite coating conductive—at least aslong as the necessary stress is applied in excess of the thresholdstress to permit tunneling. In one form, the polymer matrix may be asilicone rubber and the conductive microparticles may be silver. Theconductive microparticles may include a surface with a nanoscaleroughness that enhances an electric field gradient such that, when theconductive microparticles are less than 1-5 nm apart, electrons are ableto tunnel through the polymer matrix, thereby conducting currenttherethrough. The polymer matrix of the quantum tunneling compositecoating may provide a continuous and waterproof layer. The quantumtunneling composite coating may be disc-shaped and constructed from aquantum tunneling composite sheet.

The battery may further include a gasket that insulates and separatesthe anode and the cathode. The pressure-sensitive coating may be awater-impermeable coating that extends to the gasket and fully coversthe at least one of the anode and the cathode to leave no exposedsurfaces. However, in instances in which the pressure-sensitive coatingdoes not fully cover the anode or the cathode (such that portions of theanode or the cathode would otherwise be exposed), a further waterproofseal may extend between the pressure-sensitive coating and the gasket.Accordingly, the pressure-sensitive coating with an optional additionalwaterproof seal may be used to inhibit the battery from short circuitingin a conductive fluid below the compressive stress threshold. Thewaterproof seal may be impermeable to water and comprise a poly-dimethylsiloxane material and a cross-linking agent. Moreover, it iscontemplated that, in some forms, if a waterproof seal is presentbetween the pressure-sensitive coating or quantum tunneling compositecoating and a gasket, that the material of the polymer matrix of thepressure sensitive coating or quantum tunneling composite coating andthe material of the waterproof seal may be the same material and/orthese materials may be integral with one another.

The battery may be safely ingestible into the body because thepre-determined applied stress associated with the digestive tract of thebody is below the compressive stress threshold of the pressure-sensitivecoating. Because the stress or pressure required to initiate conductionis not applied by the digestive tract, if ingested, the battery will notconduct current and, thus, damage to intestinal tissues and release ofharmful contents of the battery into the body is avoided. In some formsof the battery, in order to provide a safety margin, the compressivestress threshold required for conduction of the coating may be at leasttwice the pre-determined applied stress associated with the digestivetract of the body.

A conductive adhesive may be positioned between the pressure-sensitivecoating and the anode and/or cathode on which the pressure-sensitivecoating is received. In some forms, the conductive adhesive may comprisea conductive silver material.

To be sufficiently high as to exceed typical digestive tract appliedstresses, the compressive stress threshold may have a value between 15.5N/cm² and 19.4 N/cm².

The pressure-sensitive coating may be a pressure-gated coating,providing relatively discrete and sudden voltage response at aparticular compressive stress.

The battery may be a button battery. However, it is contemplated thatthe concepts disclosed herein may be applicable to other styles andtypes of batteries and in particular those that might be ingested orsubjected to electrically conductive liquid environments.

According to another aspect, another battery is disclosed for use inelectronic devices in which the battery is designed to be safelyingested. The battery includes an anode, a cathode and apressure-sensitive coating. The quantum tunneling composite coatingcovers at least one of the anode and the cathode and provides pressuresensitive conductive properties including a compressive stressthreshold. The battery further includes a gasket, which separates a partof the anode and the cathode. The pressure-sensitive coating provides atleast a portion of a water-impermeable coating that extends to thegasket and fully covers the at least one of the anode and the cathode toleave no exposed surfaces, thereby inhibiting the battery from shortcircuiting in a conductive fluid.

Again, the features described above may be included in this battery. Forexample, as described above, it is contemplated that the compressivestress threshold for conduction/quantum tunneling may be greater than anapplied stress associated with the digestive tract of the body.

According to another aspect, the present invention discloses a methodfor constructing a battery having an anode and a cathode for use inelectronic devices. The battery is safely ingestible into a body thatexerts a pre-determined applied stress in its digestive tract. Themethod includes applying a pressure-sensitive coating on at least one ofthe anode and the cathode. The pressure-sensitive coating providespressure sensitive conductive properties including a compressive stressthreshold. Above the compressive stress threshold, thepressure-sensitive coating is placed in a conductive state in whichelectrons are able to conduct through the pressure-sensitive coating.Further, the compressive stress threshold is greater than thepre-determined applied stress associated with the digestive tract of thebody.

In some forms, the method may further include the steps of separating atleast a part of the anode and the cathode with a gasket and forming awaterproof seal between the pressure-sensitive coating and the gasket. Afull and complete waterproof seal between the two can inhibit the anodeand the cathode of the battery from short circuiting in a conductivefluid below the compressive stress threshold, such as may occur afteringestion of conventional batteries.

In some forms, the method may further include applying a conductiveadhesive between at least one of the anode and the cathode prior to thestep of applying the pressure-sensitive coating and adhesively attachingthe pressure-sensitive coating to the anode and/or cathode via theconductive adhesive.

These and still other advantages of the invention will be apparent fromthe detailed description and drawings. What follows is merely adescription of a preferred embodiment of the present invention. Toassess the full scope of the invention, the claims should be looked toas the preferred embodiment is not intended to be the only embodimentwithin the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of a conventional button battery;

FIG. 2 is a side cross-sectional view of a button battery with a quantumtunneling composite coating (QTCC);

FIG. 3 is a top view of the QTCC button battery of FIG. 2;

FIG. 4 is a schematic of a quantum tunneling composite coating in aninsulating state in accordance with the present invention in which thecompressive stress threshold for conduction has not been met;

FIG. 5 is a perspective view of a quantum tunneling composite coating ina conductive state in which the compressive stress threshold forconduction has been met by the applied stress;

FIG. 6a is a side cross-sectional view of the QTCC button batterycompressed below a compressive stress threshold of the QTCC;

FIG. 6b is a side cross-sectional view of the QTCC button batterycompressed above the compressive stress threshold of the QTCC;

FIG. 6c is a side view comparing the QTCC button battery compressedbelow the compressive stress threshold of the QTCC to the QTCC buttonbattery compressed above the compressive stress threshold of the QTCC;

FIG. 7a is a graph of position plotted against stress and voltage of theconventional button battery that shows the compressive stress thresholdat which the battery first achieves conduction in a dry environment;

FIG. 7b is a graph of position plotted against stress and voltage of theconventional button battery that shows a reduction in conduction voltagedue to a short-circuit current leaked via a conductive fluid (simulatedintestinal fluid) connecting an anode and cathode as an alternateconduction pathway in the conductive fluid environment;

FIG. 8a is a graph of position plotted against stress and voltage of theQTCC button battery which shows a maximum voltage is reached and isequal to the maximum voltage of the conventional button battery of FIG.7a in the dry environment;

FIG. 8b is a graph of position plotted against stress and voltage of theQTCC button battery which shows that the QTCC button battery requiressimilar levels of the compressive stress threshold to achieve conductioncompared to the conventional button battery of FIG. 7b in the simulatedintestinal fluid environment;

FIG. 9 is a side perspective view of an orally ingestible tablet used todetermine the compressive stress threshold of the QTCC button battery;

FIG. 10 is a bar graph showing the compressive stress thresholdsrequired in order to induce current flow for a conventional buttonbattery, a QTCC button battery in the dry environment, and a QTCC buttonbattery in the conductive fluid environment;

FIG. 11 is a schematic illustrating the conventional button battery inthe esophagus of a body in which the battery is short circuiting andcausing electrolysis to tissue of the esophagus;

FIG. 12 is a schematic illustrating the QTCC button battery in theesophagus of the body that was safely ingested;

FIG. 13a is an image illustrating a lumen of the esophagus with tissuedamage caused by leakage of the conventional button battery's contentswithin the conductive fluid conditions of the esophagus after ingestionof the conventional button battery;

FIG. 13b is a microscopic image of FIG. 13a illustrating necrosis andneutrophilic infiltrate of the esophagus tissue caused by leakage of theconventional button battery's contents within the esophagus afteringestion of the conventional button battery;

FIG. 13c is an image illustrating the lumen of the esophagus free oftissue damage within the conductive fluid conditions of the esophagusafter ingestion of the QTCC button battery;

FIG. 13d is a microscopic image of FIG. 13c illustrating the lumen ofthe esophagus having no tissue damage after the QTCC button battery wassafely ingested; and

FIG. 14 is a series of endoscopic images acquired at 30 minute timeintervals of the conventional button battery and the QTCC button batteryin the esophagus in vivo.

DETAILED DESCRIPTION

Referring first to FIG. 1, a conventional button battery 10 is shown incross section taken through the central axis of the battery. Theconventional button battery 10 includes an anode 14 that is generallycylindrical in shape and a cathode 16 that is also generally cylindricalin shape. The anode 14 and the cathode 16 are separated by a separator20. The separator 20 creates a barrier between the anode 14 and thecathode 16 that prevents them from touching, while still permittingelectrical charge to flow freely between them via an electrolyte. Agasket 18 also separates a part of the anode 14 and a part of thecathode 16. The gasket 18 can be an electrically insulating ring,usually polymeric, that forms a seal on a part of the anode 14 and apart of the cathode 16. In the form illustrated, the gasket 18 forms aring around the anode 14.

Although not specifically illustrated in the schematics of the batteryin FIGS. 1 through 3, which are somewhat simplified, it will beappreciated that in a commercial button battery there is typically alsoa cell top on the anode 14 and a cell can on the cathode 16 to providean outer housing of the battery. The cell can typically extends up thecylindrical outer sides of the button battery 10 to the top of thegasket 18, while the cell top is received on the inner side of thering-like gasket 18. The cell top and cell can are thin conductivehousings that cover and contain the materials of the anode 14 andcathode 16. In order to prevent short circuiting, the cell can and thecell top are not in direct contact with one another, but rather are alsoseparated from and electrically insulated from one another by theintermediate gasket 18.

It is contemplated that the cathode-anode materials of the battery 10may be any of a number of electrochemical systems including, but notlimited to, manganese dioxide-zinc, silver oxide-zinc, oxygen-zinc,manganese dioxide-lithium, carbon monofluoride-lithium, and copperoxide-lithium. Of course, other cathode or anode materials may be usedand various electrolyte materials may be used in order to provide thedesired electrical output when the button battery 10 is placed in anelectrical device for use.

Turning now to FIGS. 2 and 3, an improved button battery 12 with aquantum tunneling composite coating (QTCC) 22 is illustrated thatmodifies the structure of the conventional battery 10 illustrated inFIG. 1. The QTCC button battery 12 addresses the potential risksfollowing ingestion of the conventional button battery 10. The QTCCbutton battery 12 is a pressure sensitive, waterproof button batterydesign that adds a quantum tunneling composite coating 22 to theconventional button battery 10. As with the conventional battery 10, inthe form illustrated, the QTCC button battery 12 includes an anode 14that is generally cylindrical in shape and a cathode 16 that isgenerally cylindrical in shape. The anode 14 and the cathode 16 areagain separated by a separator 20, and a gasket 18 separates a part ofthe anode 14 and a part of the cathode 16.

In addition to the conventional button battery structure, the improvedbutton battery 12 includes a quantum tunneling composite coating 22 thatcovers at least a portion of at least one of the anode 14 and thecathode 16 in order to impart pressure sensitive conductive propertiesto the QTCC button battery 12. In the form illustrated, the quantumtunneling composite coating 22 covers a portion of the anode 14;however, in other designs, the quantum tunneling composite coating 22may cover at least a portion of the cathode 16 or may cover at least aportion of both the anode 14 and the cathode 16. The quantum tunnelingcomposite coating 22 may be disc-shaped, as shown in FIGS. 2 and 3, andconstructed of quantum tunneling composite sheets, which will bedescribed in detail below. Because the quantum tunneling compositecoating 22 is selectively conductive above a threshold applied stress(as will be described in greater detail below), it should be disposed onan outer surface of the button battery 12 to which pressure may beapplied when the battery is placed in use.

As illustrated, a conductive adhesive or paste 28 affixes the quantumtunneling composite coating 22 to the anode 14 (or the correspondingcell cap of the anode 14). If the quantum tunneling composite coating 22covers the cathode 16, then the conductive adhesive 28 may also bepositioned between the cathode 16 and the quantum tunneling compositecoating 22. However, because the conductive adhesive 28 is notconductively pressure sensitive (and always is capable of conductingcurrent), the conductive adhesive should not place the anode 14 and thecathode 16 in electrical communication with one another, therebyavoiding the creation of a direct short between the anode 14 and thecathode 16. The conductive adhesive 28 may comprise a conductive silvermaterial to facilitate the transport of electrons from the anode 14 orcathode 16 to the quantum tunneling composite coating 22 receivedthereon.

Further, a waterproof seal 30 extends between the quantum tunnelingcomposite coating 22 and the gasket 18, thereby separating a part of theanode 14 and the cathode 16 so that the QTCC button battery 12 will notshort circuit in a conductive fluid (such as intestinal fluids) below acompressive stress threshold required for the quantum tunnelingcomposite coating 22 to conduct. The waterproof seal 30 can be awater-impermeable silicone seal that is an electrically insulating,transparent silicone such as poly(dimethyl siloxane) (PDMS). Otherpossible coatings for the waterproof seal 30 could include, but are notlimited to, siloxanes, butyl rubbers, or hard thermoplastic or thermosetpolymers.

Turning now to FIGS. 4 and 5, the material of the quantum tunnelingcomposite coating 22 is schematically shown in an insulating state and aconducting state, respectively. The quantum tunneling composite coating22 comprises conductive microparticles 24 suspended in a polymer matrix26 (generally depicted as filling the volume 26). In one form, theconductive microparticles 24 can be silver and the polymer matrix 26 canbe an insulating PDMS. The depictions of FIGS. 4 and 5 are obviouslygrossly exaggerated for purposes of the schematic and it will beappreciated that far more microparticles will separate one side of thequantum tunneling composite coating 22 from the other side.

As depicted in FIG. 4, when an applied stress (σ) is zero or is lessthan the compressive stress threshold (σ_(c)) for conduction of thequantum tunneling composite coating 22, the conductive microparticles 24are sufficiently spaced to prevent the quantum tunneling of electronsthrough the polymer matrix 26. Accordingly, when insufficient stress orpressure is applied to the quantum tunneling composite coating 22 (thatis, the electrons are unable to tunnel through the quantum tunnelingcomposite coating 22), the quantum tunneling composite coating 22 isunable to conduct current therethrough and is in an insulating state.

However, as depicted in FIG. 5, when an applied stress (σ) equals orexceeds the compressive stress threshold (σ_(c)) for conduction of thequantum tunneling composite coating 22, the conductive microparticles 24are in sufficiently close proximity to enable the quantum tunneling ofelectrons through the polymer matrix 26. When electrons are able totunnel through the coating 22, the quantum tunneling composite coating22 is able to conduct current and may be said to be in a conductingstate.

Accordingly, the quantum tunneling composite coating 22 can beselectively and reversibly transformed from an insulating state (FIG. 4)to a conductive state (FIG. 5), or vise-versa, by the application ofstress or pressure which elastically deforms the polymer matrix 26 andalters the spacing of the microparticles 24 relative to one another.This mechanism allows the quantum tunneling composite coating 22 to beinsulating below and conductive above the compressive stress thresholdat which the QTCC button battery 12 achieves maximum voltage, whileremaining impermeable to water.

In addition, it is contemplated that the pressure at which the QTCCbutton battery 12 conducts is directly proportional to the thickness ofthe quantum tunneling composite coating 22 for a given density ofconductive microparticles 24. For example, when the conductivemicroparticles 24 have a higher average spacing at zero stress, greateraxial compression is required to bring the conductive microparticles 24into close enough proximity to achieve conduction. Since the quantumtunneling composite coating 22 may be affixed to the rigid housing ofthe QTCC button battery 12, compression is restricted to the axialdirection. Therefore, the required pressure for conduction of the QTCCbutton battery 12 is not diameter dependent, and can be applied to anydiameter button battery without significant design modification.Moreover, given that conduction is directly proportional to thethickness of the quantum tunneling composite coating 22 and the densityof the conductive microparticles 24, significant tunability exists fortriggering of conduction of the QTCC button battery 12.

The conductive microparticles 24 can have a nano-scale roughness presenton the surface which further enhances the electric field gradient suchthat, when the conductive microparticles 24 come into close contact (forexample, less than 1-5 nm), electrons can tunnel through the polymermatrix 26 that separates the conductive microparticles 24 in order toconduct current, shown in FIG. 5. Given that quantum tunneling does notrequire contact between the conductive microparticles 24, the polymermatrix 26 can remain continuous and therefore waterproof.

It is contemplated that other materials may also be used to makepressure-sensitive conduction coatings such as the quantum tunnelingcomposite coating 22. For example, rather than silver, themicroparticles might be made of gold particles, carbon particles, orother conductive microparticles. Likewise, the polymer matrix does notnecessarily need to be PDMS. However, it will be appreciated that theselection of materials will contribute to the spacing required in orderto create the selective pressure-sensitive conduction properties of thecoating as well as establish the compressive threshold pressure requiredfor conduction through the layer or coating.

Turning now to FIGS. 6a, 6b and 6c , the compressive stress threshold(σ_(c)) at which QTCC button batteries 12 achieve their maximum voltage(V=V_(max)) is shown. In order to determine the compressive stressthreshold at which the QTCC button battery 12 achieves its maximumvoltage, load and voltage are measured continuously between an insulatedelectrode 38 and a ground electrode 40 held in a water bath. Compressivestress and voltage are measured in real time. The QTCC button battery 12is compressed below and above the compressive stress threshold (σ_(c)),and when the circuit is completed maximum voltage (V_(max)) is achieved.Below the compressive stress threshold (σ_(c)), the QTCC 22 component ofQTCC button battery 12 remains in an insulated and non-conductive stateas illustrated in FIG. 6a and the leftmost configuration of FIG. 6c .Above the compressive stress threshold (σ_(c)), the conductivemicroparticles 24 within the QTCC 22 are in a conductive state asillustrated in FIG. 6b and the rightmost configuration of FIG. 6 c.

It will further be appreciated that the polymer matrix 26 and waterproofseal 30 may fully cover one of the surfaces of the anode 14 or cathode16 to waterproof it so that the respective anode 14 or cathode 16 has noexposed surfaces. This is perhaps better illustrated in the schematic ofFIG. 2 in which the waterproof seal 30 fully extends from the end of thequantum tunneling composite coating 22 to the gasket 18. Creating thiswaterproof seal eliminates short circuit current paths from the anode 14to the cathode 16 if the battery 12 is submerged, while still enablingthe pressure-sensitive QTCC button battery 22 to transmit its fullpotential difference (V_(max)) under sufficiently great stresses orpressures (even in the presence of a conductive fluid).

Turning now to FIGS. 7a through 8b , simultaneous stress and voltagerecordings from conventional button batteries 10 (FIGS. 7a and 7b ) andQTCC button batteries 12 (FIGS. 8a and 8b ) are shown. FIGS. 7a and 8adepict tests performed in dry environments, while FIGS. 7 b and 8 bdepict tests performed with the batteries immersed in simulatedintestinal fluid.

In FIG. 7a , in which a conventional battery is tested in a dryenvironment, under even a minimal applied stress (approximately 1 N/cm²,highlighted by the dashed line), the conventional battery achievesconduction. This stress corresponds with the stress for formingsufficient contact with the anode and cathode and immediately yieldsconduction at the maximum voltage output V_(max) of 1.5 V.

In FIG. 7b , the same conventional button battery is placed in simulatedintestinal fluid. The conventional button battery shows an approximately20 percent reduction in conduction voltage due to the short-circuitcurrent leaked via the conductive fluid now electrically connecting theanode and the cathode as an alternative conduction pathway.

However, turning now to FIGS. 8a and 8b , the QTCC button batteries 12require greater compressive stress in order to create conduction.Indeed, the compressive stress needed to initiate conduction isapproximately 50 times that of conventional button batteries 10, whichcan be shown by comparing FIGS. 7a to 8a , in a dry environment. Asshown in FIG. 8a , maximum voltage (V_(max)) is sustainably andconsistently reached at approximately 58 N/cm² by a QTCC button battery12 in the dry environment.

Even when the QTCC button battery 12 is tested in the simulatedintestinal fluid, as shown in FIG. 8b , once the compressive stressthreshold is met, the QTCC button battery 12 conducts with the samevoltage as either the conventional button battery 10 in a dryenvironment or the QTCC button battery in the dry environment.Additionally, the maximum voltage (V_(max)) achieved isindistinguishably different from that in the dry environment, as shownin FIG. 8a , due to the waterproof design of QTCC button battery 12.This establishes that in the QTCC button battery 12 little or no currentleak occurs. Thus if the button battery 12 is ingested, the generationof external electrolytic currents may be avoided.

The above demonstrates that incorporation of the QTCC 22 comprising theQTCC button battery 12 does not compromise the conductive state ofbutton batteries. In addition, once coated with QTCC 22, the QTCC buttonbattery 12 retains the capacity to power a device and through thedifferential pressure, triggering of current transmission can bemodulated. Unlike conventional button batteries 10, QTCC buttonbatteries 12 retain their voltage and output current when submerged in aconductive fluid, increasing their safety if ingested into the body andexpanding their application range to include conductive fluidenvironments.

In order to design the QTCC button battery 12 that could be safelyingested into the body, a pre-determined applied stress from the body'sdigestive tract (i.e., a human gastrointestinal (GI) tract) needs to beidentified. However, direct measurements of the GI crush strength arelacking. Therefore, extrapolated crush strength (σ*) values frompublished studies on GI crush force of humans and canines was used.

Further to quantify GI crush force, manufactured orally ingestibletablets, shown in FIG. 9, composed of Teflon® microparticles compressedaxially within cylindrical molds across the thickness of the tablets areused. As the applied compressive force (F) is increased, themicroparticles are melded into closer proximity yielding strongertablets. The applied compressive stress (σ.) was calculated by dividingthe reported compressive force (F) and cross-sectional area (A) of thetablet. Induced strain (ε) was calculated by dividing the reportedchange in tablet thickness (Δt) as compressive force was increased bythe initial tablet 36 thickness (t_(o)). From the calculated values ofstress and strain, the Young's modulus (E) of the tablet can becalculated by dividing stress by strain (E=σ/ε).

Determination of the pre-determined applied stress (that is, the GIcrush strength) and comparison to QTCC button battery 12 compressivestress threshold (σ_(c)) at which maximum voltage is achieved was doneusing a tablet crush force test using the manufactured tablet 36,illustrated in FIG. 9.

The tablet crush force testing (F*) was performed in the longitudinaldirection along the diameter of the tablets 36 as shown in FIG. 9. Tocalculate the rectangular area (A*) about which F* was acting duringtablet 36 fracture, the tablet 36 was assumed to be homogenous.Therefore Young's modulus at the point of tablet fracture (E*) can beexpressed as

${E^{\star} \approx \frac{F^{\star}l_{0}}{2t\sqrt{\frac{l_{0}\Delta\; l^{\star}}{2}\Delta\; l^{\star}}}},$where Δl* is the amount of compression the tablet underwent to failure,l_(o) is the diameter of the tablet, t is the thickness of the tablet36. Solving for Δl* yields

${\Delta\; l^{\star}} \approx {\sqrt[3]{\frac{F^{\star}l_{0}}{2t^{2}E^{\star}}}.}$Having calculated Δl*, using the Pythagorean Theorem the half-width (a*)of the flattened rectangular portion of the tablet 36 circumference canbe calculated:

$a^{\star 2} = {\left( \frac{l_{0}}{2} \right)^{2} - {\left( {\frac{l_{0}}{2} - \frac{\Delta\; l^{\star}}{2}} \right)^{2}.}}$Given that Δl* is small relative to l_(o), the Δl*² term can beconsidered negligible, therefore the equation simplifies to

$a^{\star} \approx {\sqrt{\frac{l_{0}\Delta\; l^{\star}}{2}}.}$Thus A* can be calculated: A*=2a*t. Finally, the crush strength of thetablets can be calculated as σ*≈F*/A*.

In humans, tablets 36 with a crush strength (σ*) of approximately 15.5N/cm² were crushed, while those with a crush strength of approximately24.2 N/cm² remained intact. In canines, tablets 36 with a crush strength(σ*) of 19.4 N/cm² were crushed, while those with a crush strength of23.4 N/cm² remained intact. Since the tablets 36 were similar in sizeand shape to button batteries, the calculated values serve as reliableestimates of the pre-determined applied stress (i.e., maximum crushstrength) that the GI could impose on an ingested button battery. Thepre-determined applied stress that is applied by the GI tracts of humansand dogs are less than the stresses required to create conduction in theQTCC button battery 12.

Turning now to FIG. 10, the QTCC button batteries 12 are illustrated asrequiring statistically significantly higher compressive stressthresholds to induce current flow than conventional button batteries 10in dry environments and in the simulated intestinal fluid. Furthermore,FIG. 10 illustrates that the QTCC button batteries 12 requireapproximately an order of magnitude greater compressive stress thanthose measured in the esophagus to achieve conduction. Becauseconventional button batteries 10 conduct in simulated intestinal fluidwhile the pressure sensitive QTCC button batteries 12 require stress inexcess of that experienced in the GI of adult humans even in rarespastic motility disorders, QTCC button batteries 12 may drasticallylessen or even eliminate the button battery short circuiting followingingestion. Moreover, QTCC button batteries 12 can maintain theirintegrity even when exposed to simulated intestinal fluid, whereasconventional button batteries 10 conduct sufficient electrolytic currentto degrade the conventional battery gaskets leading to battery contentrelease.

FIG. 10 further illustrates yield strengths of tablets 36 of varyingmoduli for humans and dogs. The first dashed line 41 depicts the highestyield strength tablets 36 the human esophagus was able to crush. Thesecond dashed line 42 depicts the highest yield strength tablets 36 thehuman gastrointestinal (GI) tract was able to crush. The third dashedline 44 depicts the highest yield strength tablets 36 the canine GItract was able to crush. The fourth dashed line 46 represents the yieldstrength of tablets 36 that were sufficiently strong to survivedigestion in humans and dogs while remaining intact. Notably, theaverage compressive stress threshold for QTCC button batteries 12 isapproximately twice the yield strength of tablets 36 that remainedintact throughout digestion by dogs or humans, which indicates that QTCCbutton batteries 12, unlike conventional button batteries 10, will notconduct a current (assuming they are also made to be waterproof to avoidshort-circuit after ingestion).

Turning now to FIG. 11, a schematic of an ingested conventional buttonbattery 10 injuring the esophagus 48 is shown. When conventional buttonbatteries 10 contact or are pressed against GI mucosal tissues of theesophagus 48 or small intestine 49, for example, their short circuitcurrent and electrolysis cause injury to the surrounding tissue.However, as shown in FIG. 12, the QTCC button batteries 12 (and for thereasons detailed above) do not short circuit either in contact with oras a result of pressure produced by GI motility, thereby increasingsafety if ingested.

In addition to the acute health risks of short circuit current in theGI, the potential for absorption of the heavy metal contents ofconventional button batteries 10 pose long-term health consequences.This is of particular importance in the pediatric community where theingestion of button batteries occurs most frequently and may compromiseneurological development. QTCC button batteries 12 remain intact inconductive fluid conditions after 48 hours showing no indication ofshort circuit current loss or expelling of toxic contents.

FIGS. 13a, 13b, 13c, and 13d further illustrate the QTCC button battery12 remaining intact in conductive fluid conditions within the esophagus48, while the conventional button battery 10 leaks during an in vivoesophageal battery retention model. Both the QTCC button battery 12 andthe conventional button battery 10 were repeatedly evaluatedendoscopically through the use of an esophageal overtube (not shown) toensure the batteries remained stationary, thus approximating esophagealbattery retention. The conventional button battery 10 and the QTCCbutton battery 12 were deployed in the esophagus 48 with the anodepositioned on the posterior aspect of the esophagus 48 of the animal inthe supine position.

As shown in FIG. 13a , the conventional button battery 10 shows leakageof battery acid over the course of 2 hours during the endoscopicevaluation. Microscopically, as shown in FIG. 13b , esophageal tissueexposed to the conventional button battery 10 for 2 hours exhibitednecrosis, as evidenced by desquamation and paucity of nuclei in theesophagus epithelium. The desquamation and paucity of nuclei is shownwithin the dashed line 50 in FIG. 13b . Additionally, the esophagealtissue exposed to the conventional button battery 10 exhibitedneutrophilic infiltrate, as evidenced by the arrows 52 in FIG. 13b . Incontrast, as shown in FIGS. 13c and 13d , the esophagus 48 continuouslyexposed to the QTCC button battery 12 appears normal bothmacroscopically and microscopically, thereby indicating no tissuedamage.

Further, FIG. 14 shows endoscopic images acquired every 30 minutes ofthe conventional button battery 10 and the QTCC button battery 12 in theesophagus 48 in vivo. As shown in the top section of FIG. 14, theconventional button battery 10 demonstrates leaking of its contentsstarting at the 30 minute time point and continues to leak its contentsup to the 120 minute time point. However, the QTCC button battery 12, asshown in the lower section of FIG. 14, appears to remain intactthroughout the test period from 0 minutes to 120 minutes.

The above disclosed QTCC button battery 12 has several advantages. Thematerials used to construct QTCC button batteries 12 are inexpensive andreadily scalable for mass production. Further, the QTCC button battery12 is a waterproof, pressure sensitive, button battery that isinsulating in the intestinal environment and conductive in standardbattery housings. Importantly, the QTCC button battery 12 enablesexisting devices to be powered without modification, as these devicesare typically capable of providing a compressive stress above thecompressive stress threshold for conduction.

In addition, electromechanical characterization demonstrates that QTCCbutton batteries 12 can require approximately twice the pre-determinedapplied stress supplied by the adult gastrointestinal tract to conduct,providing a sufficient margin of safety given the potential variabilityof digestive tracts. While conventional button batteries 10 that areimmersed in the conductive fluid environment readily damage intestinaltissue and rapidly release harmful contents including cadmium, lead,mercury and lithium that may be absorbed, immersion of QTCC buttonbatteries 12 under the same conditions remain waterproof and do notproduce electrolysis or tissue damage following immersion.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

The invention claimed is:
 1. A method of making a battery, comprising:applying a pressure-sensitive coating that comprises a quantum-tunnelingcomposite coating on at least one of an anode and a cathode of abattery, the pressure-sensitive coating having pressure sensitiveconductive properties including a compressive stress threshold abovewhich the quantum-tunneling composite coating is placed in a conductivestate in which electrons are able to conduct through thequantum-tunneling composite coating, wherein the compressive stressthreshold is greater than a predetermined applied stress associated witha body's digestive tract.
 2. The method of claim 1, wherein the batteryis a button battery.
 3. The method of claim 1, wherein the quantumtunneling composite coating comprises a polymer matrix with conductivemicroparticles suspended therein that collectively provide the pressuresensitive conductive properties of the quantum tunneling compositecoating.
 4. The method of claim 3, wherein the polymer matrix comprisesa silicone rubber and the conductive microparticles comprise silver. 5.The method of claim 3, wherein the conductive microparticles include asurface with a nanoscale roughness that enhances an electric fieldgradient such that, when the conductive microparticles are less than 1-5nm apart, electrons are able to tunnel through the polymer matrix,thereby conducting current therethrough.
 6. The method of claim 1,further comprising: separating at least a part of the anode and thecathode with a gasket; and forming a waterproof seal between thepressure-sensitive coating and the gasket, the waterproof sealinhibiting the anode and the cathode of the battery from shortcircuiting in a conductive fluid below the compressive stress threshold.7. The method of claim 6, wherein the waterproof seal is impermeable towater and comprises a poly-dimethyl siloxane material and across-linking agent.
 8. The method of claim 6, wherein thepressure-sensitive coating is a water-impermeable coating extending tothe gasket and fully covering the at least one of the anode and thecathode to leave no exposed surfaces.
 9. The method of claim 1, furthercomprising applying a conductive adhesive between at least one of theanode and the cathode prior to the step of applying thepressure-sensitive coating; and adhesively attaching thepressure-sensitive coating to the at least one of the anode and cathodevia the conductive adhesive.
 10. The method of claim 1, wherein thequantum tunneling composite coating is disc-shaped and constructed froma quantum tunneling composite sheet.
 11. The method of claim 1, wherein,when no stress or a stress below the compressive stress threshold isapplied to the quantum tunneling composite coating, the quantumtunneling composite coating is in an insulating state in which electronsare unable to quantum tunnel through the quantum tunneling compositecoating.
 12. The method of claim 1, wherein the battery is safelyingestible into the body because the pre-determined applied stressassociated with the digestive tract of the body is below the compressivestress threshold of the pressure-sensitive coating, thereby inhibitingdamage to intestinal tissue and inhibiting release of harmful contentsof the battery into the body.
 13. The method of claim 1, wherein thecompressive stress threshold is at least twice the pre-determinedapplied stress associated with the digestive tract of the body.
 14. Themethod of claim 1, wherein the pressure-sensitive coating also providesa continuous and waterproof layer over at least one of the anode and thecathode.
 15. The method of claim 1, wherein the compressive stressthreshold has a value between 15.5 N/cm² and 19.4 N/cm².
 16. The methodof claim 1, wherein the pressure-sensitive coating is a pressure-gatedcoating.